There has publicly been known a volumetric feeder configured to continuously discharge a constant amount of a powdery material per a predetermined time and feed another device with the powdery material (see “COPERION K-TRON (registered trademark (international registration)) volumetric feeder”, (online), Apte Inc., (searched on Jan. 27, 2017), “Technical movies on law of circle feeder”, (online), YOSHIKAWA Corporation, (Searched on Jan. 28, 2017).
The volumetric feeder is configured to deliver a powdery material dropping from a hopper reserving the powdery material by a screw feeder, a table feeder, a circle feeder (registered trademark), a rotary feeder, or the like. The volumetric feeder discharges the powdery material having a discharge flow rate per unit time, which is measured by a measuring instrument such as a load cell. A motor functioning as a drive source of the screw feeder or the like is feedback controlled to allow the discharged powdery material to have a target flow rate.
The hopper needs to timely be supplied with a powdery material because the powdery material reserved in the hopper gradually decreases in amount as the powdery material is discharged. When the hopper is supplied with a required powdery material, the powdery material has a load or an impact that is transmitted to the measuring instrument and thus affects an output signal of the measuring instrument. When the flow rate of the discharged powdery material is calculated in accordance with such an output signal, the calculated discharge flow rate may deviate from an actual discharge flow rate and the motor of the screw feeder or the like may have a rotational speed or output torque erroneously corrected in accordance with the discharge flow rate including an error.
Conventionally, feedback control according to an output signal of the measuring instrument is thus halted temporarily while the hopper is supplied with a powdery material and the motor has a rotational speed kept at the value just before the halt of the feedback control, to allow the discharged powdery material to be kept at the target flow rate.
However, the flow rate of the discharged powdery material is not definitely kept at the target flow rate, although the rotational speed of the motor is kept at the value just before the halt of the feedback control.